1. Field of the Invention
The present invention relates to a planographic printing plate that can be written with an infrared laser and has a highly sensitive negative photosensitive layer, and to a positive planographic printing plate that can be written by heat from an infrared laser, a thermal head or the like, particularly a positive planographic printing plate for so-called direct plate formation in which a plate can be formed directly by scanning an infrared laser based on digital signals from a computer or the like.
2. Description of the Invention
The development of lasers in recent years has been remarkable. In particular, high-output, compact solid-state lasers and semiconductor lasers having an emission range in the near infrared to infrared range are being developed. Accordingly, these lasers are extremely useful as an exposure light source at the time a printing plate is formed directly on the basis of digital data from a computer or the like.
A negative planographic printing plate material for an infrared laser, which negative planographic printing plate material uses as an exposure source an infrared laser having an emission range in the infrared range is a planographic printing plate material having a photosensitive layer containing an IR absorber, a polymerization initiator generating radicals by light or heat, and a polymerizable compound.
Usually, such a negative image-recording material utilizes a recording system where radicals generated by light or heat are used as the initiator to generate a polymerization reaction for hardening the photosensitive layer of exposed portions to form an image portion. Because this negative image-forming material is inferior in image formability when compared to the positive which causes dissolution of the photosensitive layer by the exposure energy of an infrared laser, heat treatment is generally conducted before the development step in order to promote a hardening reaction by polymerization thereby forming a strong image portion.
As a printing plate having a photosensitive layer polymerizable by light or heat, techniques are known in which a light-polymerizable or heat-polymerizable composition is used as a photosensitive layer, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 8-108621. In preparing a planographic printing plate using such an image recording material, however, there is the problem that as described in said publication, stable and highly sensitive images cannot be obtained without conducting pre-heating and post-heat treatment in order to stabilize the images. This is possibly because not all energy from an infrared laser for recording is used for decomposition of the polymerization initiator, thus lowering the efficiency of decomposition.
On the other hand, a positive planographic printing plate material for infrared lasers for direct plate making is disclosed in JP-A No. 7-285275. This prior art invention relates to an image-recording material comprising a material generating heat upon absorption of light and a positive photosensitive compound such as quinonediazide etc. added to a resin soluble in an aqueous alkali solution, wherein the positive photosensitive compound acts, in the image portion, as a dissolution inhibitor for substantially lowering the solubility of the resin soluble in an aqueous alkali solution, while in the non-image portion, the positive photosensitive compound is decomposed by heat and removed by development thus preventing it from exhibiting the dissolution-inhibiting properties and images are thereby formed.
As a result of investigation, the present inventors found that positive images can be obtained even if the quinonediazide or the like is not added to the image recording material, but there is the problem that the image-recording material to which the quinonediazide or the like is not added the stability of sensitivity with respect to the concentration of a developing solution, that is the development latitude, is poor. The development latitude refers to an allowable range in which good images can be formed when the alkali concentration in an alkali developing solution is changed.
On the other hand, onium salts or alkali-insoluble compounds capable of hydrogen bonding are known to have an alkali dissolution-inhibiting action on resins soluble in an aqueous alkali solution. WO 97/39894 describes a infrared laser-compatible image-forming material in which a composition which uses a cationic, infrared absorbing dye as dissolution inhibitor of a resin soluble in an aqueous alkali solution, behaves like a positive. This behavior is one in which the infrared absorbing dye absorbs laser rays to generate heat thereby eliminating the effect of inhibiting the dissolution of a polymeric film on the exposed portion, and thereby carrying out image formation.
In these positive image-forming materials, the interaction among polymers constituting the photosensitive layer is eliminated by exposure for image formation, and thus control of interaction among the polymers is important. In the planographic printing plate provided with such a heat mode-compatible photosensitive layer, however, there is the problem that the interaction among the polymers is varied with time, and the coating properties of the photosensitive layer are lowered and the image forming properties vary.